The present invention relates to positioning tables and more particularly to techniques for controlling the tendency of retainers of positioning table bearings to creep from their ideal positions relative to their path of travel.
The use of positioning tables to enable movement of workpieces or the like back and forth along an axis at controlled rates for precise positioning is well established. Such positioning tables are often used in vertical or slanted positions as well as in apparently level positions.
One type of positioning table employs V-grooves in a U-shaped base that face corresponding V-grooves in the edges of a slide. Ball or roller bearings ride in the V-grooves to permit the slide to move with respect to the base. The base applies a large preload to the bearings to ensure respectable travel of the slide. The bearings on each side of the slide are conventionally held in desired relative positions by a retainer.
The V-grooves in the foregoing device may be machined into the edges of the base and the slide, or they may be machined into separate rails which are affixed facing each other.
Other types of positioning tables employ grooves having cross-sectional shapes of gothic arches, or portions of two circles. Alternatively, instead of using machined shapes, two rods may be affixed to the base, and an additional two rods may be affixed to the base. The rolling bearing elements bear against the rods in a manner analogous to their function with machined shapes.
For purposes of the present invention, all such arrangements are equivalent. The remainder of the discussion of the background, and the detailed description of the invention, is directed to the illustrative example wherein V-grooves are machined into the facing surfaces of the base and the slide, and the rolling bearing elements are balls.
During use, the bearings and their retainer travel half the distance travelled by the slide. The limits of travel of the slide are established by the length of the slide and base. At its limits of travel, the slide overhangs the end of the base, and the end bearing is adjacent the end of the base. The total slide travel is normally equal to twice the length of the slide, minus twice the length of the bearing assembly.
A major problem is the tendency of the bearings to creep from their nominal position with respect to the slide. Such creeping reduces the available travel of the slide. This creeping may result from the use of positioning tables in a non-level position or from manufacturing imperfections such as variations in flatness and straightness of the bearing surfaces of the V-grooves, or imperfections in the bearings.
While the creep may be only a few micro-inches per end-to-end operation of the positioning table, tens of thousands of operations can produce a total creep that seriously reduces the available travel.
To correct such creep, the prior art uses a pinion engaging racks on the base and the slide. The pinion travels half the distance of slide travel, the same as the nominal travel of the bearings. The pinion is connected to the bearing retainer to force the retainer, and its bearings, back to their nominal positions with respect to the slide. This is accomplished by sliding the bearings in the ways.
With a light preload on the bearings, the rack and pinion idea may be satisfactory. However, some positioning tables employ a preload on the bearings on the order of several hundred pounds. The force required to slide the retainer and bearings back to their required positions in such a case may be two hundred pounds. This would place a large burden on the motor driving the slide. In some cases the motor may not be capable of driving the retainers back to their normal position, which may cause motor overheating or the actuation of a fuse or circuit breaker. In these cases, the operator may have to "bang" the retainers back to their normal position or to use a program in an automatic machine to push the retainers. In either case, this banging or pushing may cause premature failure of the bearings, and may cause damage or misadjustment of a sensitive load driven by the stage.